126 PROTOPLASMIC AGE OF PROTOZOA 



Polystomella crispa, in gregarines, and coccidia, where the residual 

 primary nucleus, or the Restkorperchen, may be interpreted as the 

 now functionless somatic chromatin. 



Idiochromidia, or germ plasm, therefore, must be interpreted, in 

 some cases at least (infusoria), as a definite and distinct substance of 

 the cell. In other cases its segregation and separation from somatic 

 chromatin occurs only during the second period of the life cycle, and 

 its formation is the index of advancing age (sarcodina). It is, in point 

 of fact, the chief morphological feature characteristic of the period of 

 maturity in protozoa. 



3. Sex Differentiation. — At the present time the hypothesis first 

 advanced by Montgomery ('01) is widely accepted, that during 

 maturation of the germ cells the reduced number of chromosomes is 

 brought about by union, two by two, of chromosomes representing the 

 same characteristics of the adult in maternal and paternal ancestors. 

 Of such characteristics, none are more marked than those primary and 

 secondary characters which distinguish the sexes. Wilson's obser- 

 vations, following and enlarging upon those of McClung, Stevens, 

 and others, on the structure of the germ nuclei in insects, have prac- 

 tically demonstrated that sex here, like other adult characteristics, is a 

 matter of inheritance. 



In protozoa, sex differentiation, when present, is, apparently, the 

 final expression of the period of maturity. We have seen that, with 

 advancing age, the structure of the protozoan cell may become materi- 

 ally altered, and that these alterations may give rise to similar con- 

 jugating gametes, or, directed possibly by inheritance, may give rise 

 to male or female germ cells. In the former case (isogamy), conju- 

 gating elements may be similar in size to normal cells or only slightly 

 reduced, as in paramecium, didinium, and the majority of infusoria; 

 or both may be reduced to small-sized equal cells (isomicrogametes), 

 as in many gregarines and rhizopods. In the latter case (anisogamy) 

 one cell, macrogamete, may be similar to the ordinary vegetative cells 

 (as in vorticella, coccidium, etc.), or only slightly changed, while the 

 other cell (microgamete) may be relatively minute (vorticellidse, 

 coccidiidia, etc.) ; or both cells may be reduced and of dissimilar size 

 (as in polytoma, centropyxis, schaudinnella, stylorhynchus, and other 

 gregarines). 



In sexually dimorphic gametes there is no difference between the 

 early cells in the majority of cases, differentiation coming only as a 

 last step in maturity (hemosporidia, coccidium, and coccidiidia gener- 

 ally); in some cases, however, notably in adelea (Siedlecki, 1S99) and 

 cyclospora (Schaudinn, 1902) among coccidia, and in trypanosoma 

 (Schaudinn, 1904) among flagellates, the sex dift'erences are said to 

 extend as far back as the schizont stage immediately after fertiliza- 

 tion; hence, if this is true, it is possible to speak in some cases of male 



